1. What does it mean?
입틀막 shortens 입을 틀어막다. In reaction captions it evokes putting a hand over your own mouth because something is startling, impressive, or difficult to believe.
Also written or said as ipteulmak
2. The nuance
Not every use is a delighted reaction. The same expression can describe covering another person's mouth or silencing them in news and criticism. Identify who is doing the covering and whose mouth is involved.
3. Examples
“상자 안에 오래 찾던 책이 있어서 입틀막했어.”
I covered my mouth in surprise when I found the book I'd been looking for inside the box.
“마지막 장면은 스포 없이 봐. 나 진짜 입틀막했어.”
Watch the last scene without spoilers. It genuinely left me speechless.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.